Comparison of biliary excretion of iodothyronines after intravenous and intraduodenal administration of various doses of T4 and T3.

Intraduodenal 10 or 40 micrograms thyroxine (T4) and 5 or 10 micrograms triiodothyronine (T3) or i.v. bolus of 2, 4 or 8 micrograms T4 or infusion of 10 micrograms T4 within 2 h were administered to groups of 3 to 5 rats. All experiments were made under pentobarbiturate anesthesia and administration of heparin and changes of several iodothyronine levels in plasma and of their biliary excretion were measured with the aid of specific radioimmunoassay. About 1 to 4 per cent of administered T4 were found in bile within 4 h. From this it was concluded that hepatic clearance of T4 does not increase significantly under nearly physiological levels of this hormone in plasma and that, under such conditions, a majority of absorbed or administered T4 is distributed and utilized within the whole body rather than excreted by bile. In addition, several changes of the level of various iodothyronines in plasma and of their excretion by bile as related to the administered doses of T4 and T3 were described.